/*
 * Copyright (C) 2009 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.util.concurrent;

import java.util.concurrent.Executor;
import java.util.concurrent.Future;

import com.google.common.base.Service;
import com.google.common.base.Throwables;

/**
 * Base class for services that can implement {@link #startUp}, {@link #run} and
 * {@link #shutDown} methods. This class uses a single thread to execute the
 * service; consider {@link AbstractService} if you would like to manage any
 * threading manually.
 * 
 * @author Jesse Wilson
 * @since 2009.09.15 <b>tentative</b>
 */
public abstract class AbstractExecutionThreadService implements Service {

    /* use AbstractService for state management */
    private final Service delegate = new AbstractService() {
        @Override
        protected final void doStart() {
            executor().execute(new Runnable() {
                public void run() {
                    try {
                        startUp();
                        notifyStarted();

                        if (isRunning()) {
                            try {
                                AbstractExecutionThreadService.this.run();
                            } catch (Throwable t) {
                                try {
                                    shutDown();
                                } catch (Exception ignored) {
                                }
                                throw t;
                            }
                        }

                        shutDown();
                        notifyStopped();
                    } catch (Throwable t) {
                        notifyFailed(t);
                        throw Throwables.propagate(t);
                    }
                }
            });
        }

        @Override
        protected void doStop() {
            triggerShutdown();
        }
    };

    /**
     * Start the service. This method is invoked on the execution thread.
     */
    protected void startUp() throws Exception {
    }

    /**
     * Run the service. This method is invoked on the execution thread.
     * Implementations must respond to stop requests. You could poll for
     * lifecycle changes in a work loop:
     * 
     * <pre>
     *   public void run() {
     *     while ({@link #isRunning()}) {
     *       // perform a unit of work
     *     }
     *   }
     * </pre>
     * 
     * ...or you could respond to stop requests by implementing
     * {@link #triggerShutdown()}, which should cause {@link #run()} to return.
     */
    protected abstract void run() throws Exception;

    /**
     * Stop the service. This method is invoked on the execution thread.
     */
    // TODO: consider supporting a TearDownTestCase-like API
    protected void shutDown() throws Exception {
    }

    /**
     * Invoked to request the service to stop.
     */
    protected void triggerShutdown() {
    }

    /**
     * Returns the {@link Executor} that will be used to run this service.
     * Subclasses may override this method to use a custom {@link Executor},
     * which may configure its worker thread with a specific name, thread group
     * or priority. The returned executor's {@link Executor#execute(Runnable)
     * execute()} method is called when this service is started, and should
     * return promptly.
     */
    protected Executor executor() {
        return new Executor() {
            public void execute(Runnable command) {
                new Thread(command,
                        AbstractExecutionThreadService.this.toString()).start();
            }
        };
    }

    @Override
    public String toString() {
        return getClass().getSimpleName();
    }

    // We override instead of using ForwardingService so that these can be
    // final.

    /* @Override */public final Future<State> start() {
        return delegate.start();
    }

    /* @Override */public final State startAndWait() {
        return delegate.startAndWait();
    }

    /* @Override */public final boolean isRunning() {
        return delegate.isRunning();
    }

    /* @Override */public final State state() {
        return delegate.state();
    }

    /* @Override */public final Future<State> stop() {
        return delegate.stop();
    }

    /* @Override */public final State stopAndWait() {
        return delegate.stopAndWait();
    }
}
